RE: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: Wednesday, January 30, 2013 3:53 AM
> To: KY Srinivasan
> Cc: olaf@aepfle.de; bp@alien8.de; apw@canonical.com; x86@kernel.org;
> tglx@linutronix.de; devel@linuxdriverproject.org; gregkh@linuxfoundation.org;
> jasowang@redhat.com; linux-kernel@vger.kernel.org; hpa@zytor.com
> Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> 
> >>> On 30.01.13 at 01:51, "K. Y. Srinivasan" <kys@microsoft.com> wrote:
> > Xen emulates Hyper-V to host enlightened Windows. Looks like this
> > emulation may be turned on by default even for Linux guests. Check and
> > fail Hyper-V detection if we are on Xen.
> >
> > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> > ---
> >  arch/x86/kernel/cpu/mshyperv.c |    7 +++++++
> >  1 files changed, 7 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c
> b/arch/x86/kernel/cpu/mshyperv.c
> > index 646d192..4dab317 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -30,6 +30,13 @@ static bool __init ms_hyperv_platform(void)
> >  	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> >  		return false;
> >
> > +	/*
> > +	 * Xen emulates Hyper-V to support enlightened Windows.
> > +	 * Check to see first if we are on a Xen Hypervisor.
> > +	 */
> > +	if (xen_cpuid_base())
> > +		return false;
> > +
> >  	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
> >  	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
> 
> I'm not convinced that's the right approach - any hypervisor
> could do similar emulation, and hence you either want to make
> sure you run on Hyper-V (by excluding all others), or you
> tolerate using the emulation (which may require syncing up with
> the other guest implementations so that shared resources don't
> get used by two parties).
> 
> I also wonder whether using the Hyper-V emulation (where
> useful, there might not be anything right now, but this may
> change going forward) when no Xen support is configured
> wouldn't be better than not using anything...

Jan,

Presumably, Hyper-V emulation is only to run enlightened Windows. The issue with
Xen is not that it emulates Hyper-V, but this emulation is turned on while running Linux.
That is the reason I chose to check for Xen. Would you prefer a DMI check for the Hyper-V
platform. 

Regards,

K. Y